This invention relates to field emission guns, either electron or positive ion emission, and particularly to circuitry for the accurate measurement and control of the current in that part of the beam actually being used, and for producing a correction signal that may be applied to a control electrode for beam noise compensation.
Noise produced by contamination and erosion of the pointed field emission source in the high vacuum used for field emission applications results in variations in the electron or ion beam current from field emission guns. For accurate applications of such guns, this flicker noise must be eliminated or greatly reduced in that part of the beam being used. Some of the inherent noise can be reduced by refinement of cathode materials and improvement in operating conditions. It is further possible to reduce or even eliminate this flicker noise by use of feedback techniques provided that the appropriate beam current can be measured and side effects associated with de-focus and translation of the beam can be avoided.
Reduction or elimination of flicker noise by use of feedback techniques requires an accurate measurement of the beam current. In most field electron emission gun systems only a small portion of the electron beam is actually used and this small portion is directed to the sample at anode potential through a small aperture in the center of an accelerating electrode positioned near the cathode and in the path of the emitted electron beam. If the emitted beam had a uniform and constant electron density throughout, a measure of beam current could be made by sampling the current onto this accelerating anode. Unfortunately, due to an effect known as spatial correlation, the noise in the total emission current differs from that in the beam passing through the aperture in the accelerating anode so that to perform feedback noise compensation, current measurements must be made on the beam that impinges on the actual sample being exposed.
Some prior art devices recognizing the problem of spatial correlation throughout a beam, employ an annular collector from which current measurement may be made. In some prior art devices, the collector may have a small central aperture through which the central portion of the emitted beam may pass and from which a sample of beam current is obtained. Other prior art devices, such as that described in U.S. Pat. No. 3,925,706, actually measure the total usable beam current at the anode and use the measured value to generate a feedback signal to a control electrode in the vicinity of the anode. While this last described prior art method provides a measurement of beam current received at the anode, the measured current cannot always be simultaneously used, and therefore cannot provide the proper control to the beam at the cathode. Further, current measurements made at or near ground anode potential result in an error signal that must be translated to a very high negative voltage before being applied to the power supply reference to the cathode, thereby introducing additional possibility of error. Greater error occurs, however, in that the current within the emission beam is usually a small fraction of the current flowing within the high voltage loop with the remainder consisting of leakage, corona, capacitive pickup to and from the power supplies floating at high voltage, and pickup associated with the transmission of power to the supplies from ground potential. All of these items tend to introduce measurement errors between the anode and the control electrode in the vicinity of the cathode. These errors are eliminated in the circuitry described hereinafter.
Briefly described, the circuitry of the invention measures field emission current at the current emitting electrode by enclosing the emitting electrode and all of its associated power supply current measuring circuitry within a conductive shield at a local ground reference and to which all of the enclosed circuitry is thus referenced. This first shield is enclosed within, and insulated from, a second shield that is connected directly to the high voltage terminal of the accelerating voltage source which itself is referenced to earth ground. Beam current is measured between the outer or second shield at a virtual ground potential and the inner or first shield at local ground potential to produce a voltage signal that is compared with a reference level to obtain an error signal which is applied to a Wehnelt or control electrode in the vicinity of the tip of the beam current emitting electrode to thereby modulate its emission to maintain a constant beam current. By use of this double shield configuration enclosing the various circuits subject to pickup and leakage, the actual emitted beam current is readily distinguished from the spurious currents within the system.